With a small, sharp knife, peel the cucumber and slice it
lengthwise into halves. Scoop out the seeds by running the tip of a teaspoon
down the center of each half. Cut the cucumber lengthwise into 1/8-inch-thick
slices, then crosswise into ½” inch pieces.

Combine the cucumber, onions and salt in a small bowl, and mix
them together thoroughly with a spoon. Let the mixture rest at room temperature
for 5 minutes or so, then squeeze the cucumbers gently between your fingers to
remove the excess liquid.

Drop the cucumber pieces into a deep bowl, add
the tomato and coriander, and toss them together gently but thoroughly. Combine
the yoghurt and cumin and pour it over the vegetables, turning them about with
a spoon to coat them evenly. Taste for seasoning, cover tightly, and
refrigerate for at least 1 hour, or until completely chilled, before serving.